Control of the initiation of combustion and control of combustion

ABSTRACT

Method and apparatus for controlling the initiation and completion of self-sustaining combustion in fuel/oxidizer mixtures through the use of ultraviolet radiation absorbed throughout the mixture.

This is a division of application Ser. No. 468,543, filed May 9, 1974.

BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION

This invention relates to methods and apparatus for controlling theinitiation and completion of combustion in combustible mixtures. Theinvention has particular application and is directed to thephotochemical control of the initiation and completion of combustion,particularly in fuel/oxygen/diluent combustible mixtures.

Conventionally, combustible mixtures are ignited by raising thetemperature of a zone within such a mixture to the thermal ignitionpoint, so that ignition is initiated in the zone and is then propagatedthroughout the mixture. Thermal ignition temperatures of conventionalmixtures are relatively high, and at such high temperatures pollutantsare formed. Further, the creation of pollutants in the normal mixturerange has been found to decrease with decreasing proportions of fuel inthe mixture, so that fuel-lean mixtures generate less pollutants.

The present invention involves the appreciation that the irradiation ofa combustible mixture with photon energy in the ultraviolet rangepreconditions the mixture by the photodissociative creation of acombustion intermediary species. For example, in the case of afuel/oxygen mixture, additional oxygen atom concentrations arephotochemically created far in excess of those existing at thetemperature of the mixture. Such concentrations of combustionintermediary species in the mixture result in the effective lowering ofthe thermal ignition temperature of the mixing and also permitcombustion with greatly reduced proportions of fuel and at greatlyreduced pressures. Further, such ultraviolet irradiation has been foundto effect the reaction front propagation velocity and decrease ignitiondelay. All of these factors enhance the combustion process and reducepollutants generated.

Photochemical control of the combustion process is preferably to theconventional control by spark or other sources such as glow discharge,exploding wires, hot wire, and the like. In particular, initiation ofthe combustion process by these conventional devices does not readilyoffer a means for controlling the rate of combustion or flameprogagation speed as does photochemical control.

In the present invention, the photodissociative creation of a combustionintermediary species may be in a concentration below that required forthe photochemical initiation of combustion at the temperature of themixture but above the concentration of the species that would exist atthe thermal ignition temperature of the mixture. In this fashion, themixture is preconditioned so that the thermal ignition temperature issignificantly lowered and the mixture is closer to combustion. Followingthe preconditioning, combustion may be initiated by irradiating themixture with a superimposed ultraviolet flash of sufficent intensity toinitiate combustion therein. Alternatively and as another example, thepreconditioning of the mixture may be followed by a superimposedelectrical spark discharge therein or by some other heating method toinitiate combustion.

The present invention also utilizes the irradiation of a combustionmixture from a plurality of sources of ultraviolet energy, therebyenhancing the photochemical combustion process. By the use of suchplural sources, it has been found that different zones within acombustible mixture may be preconditioned and the combustion processestherein controlled to enhance the combustion process of the overallmixture.

The invention contemplates unique sources of ultraviolet energy in theform of spaced electrodes in an inert gas atmosphere. In one embodimenta window is employed to provide for the tranmission of ultravioletenergy from a sealed source, without appreciable absorption thereof,into a combustible mixture. In another embodiment, a "windowless" sourceis employed utilizing a flow of inert gas to dynamically create thenecessary gaseous conditions between the electrodes, displacing foreigngas, and thus providing for efficient generation of the necessaryultraviolet radiation.

The present invention thus has application to all combustion processes,and in particular to those combustion processes involving oxygen, e.g.,combustion processes in automotive and aircraft engines. The techniquesmay be employed in enhancing combustion in all combustion chambers,including exhaust systems of combustion devices.

The work to date in the investigation of photochemical control of thecombustion process has largely been theoretical. Representativepublications are as follows:

1. "Final Report - Photochemical Enhancement of Combustion and Mixing inSupersonic Flows", by A. E. Cerkanowicz, Photochem Industries, Inc.,Fairfield, N.J., dated Nov. 1973, distributed on Apr. 1, 1974.

2. "Interim Scientific Report - Photochemical Enhancement of Combustionand Mixing in Supersonic Flows", by A. E. Cerkanowicz, PhotochemIndustries., Fairfield, N.J., dated March 1972, distributed May 16,1973.

3. "Photochemical Ignition and Combustion Enhancement in High SpeedFlows of Fuel-Air Mixtures", by A. E. Cerkanowicz and R. F. McAlevy III,Photochem Industries, Incorporated, Fairfield, New Jersey, published byAmerican Institute of Aeronautics and Astronautics at AIAA 11THAEROSPACE SCIENCES MEETING, WASHINGTON D.C./JANUARY 1-12, 1973, AIAAPaper No. 73-216.

4. "The Photochemical Ignition Mechanism of Unsensitized Fuel-AirMixtures", By A. E. Cerkanowicz, M. E. Levy and R. F. McAlevy III,Photochem Industries, Fairfield, New Jersey, published by AmericanInstitute of Aeronautics and Astronautics at AIAA 8TH AEROSPACE SCIENCESMEETING, NEW YORK, NEW YORK/JANUARY 19-21, 1970, AIAA Paper No. 70-149.

5. "Argon Photoionization Cross-Sections and Autoionized Line Profilesin the 504-304 A Region", by M. E. Levy, Photochem Industries, Hoboken,N.J. 07030, and R. E. Huffman, Air Force Cambridge ResearchLaboratories, Bedford, Massachusetts 01731, published by Pergamon Press1969 in J. QUANT, SPECTROSE, RADIAT, TRANSFER, Vol. 9, pp. 1349-1358,Printed in Great Britain.

6. "Igniton of Subatmospheric Gaseous Fuel-Oxidant Mixtures byUltraviolet Irradiation", by M. E. Levy and A. E. Cerkanowicz, VitroLaboratories, West Orange, New Jersey and R. F. McAlevy III, StevensInstitute of Technology, Hoboken, New Jersey, published by AmericanInstitute of Aeronautics and Astronautics at AIAA 7TH AEROSPACE SCIENCESMEETING, NEW YORK CITY, NEW YORK/JANUARY 20-22, 1969, AIAA Paper No.69-88.

7. "Photochemical Ignition of Low Pressure Fuel-Oxidizer Mixtures", byM. E. Levy and A. E. Cerkanowicz, Vitro Laboratories, West Orange, NewJersey and R. F. McAlevy III, Mechanical Engineering Department, StevensInstitute of Technology, Hoboken, New Jersey, paper delivered beforefall meeting of the Western States Section of the Combustion Instituteheld at Stamford Research Center Institute, Palo Alto, California(Monday and Thuesday of third week of October 1968).

8. ROCKETS, October 1945, Page 10, (copy in class 60, subclass 39.82).

Representative patents are as follows:

    ______________________________________                                        U.S. PATENT  ISSUE DATE   PATENTEE                                            ______________________________________                                        3,190,823-204/157                                                                          June 22, 1965                                                                              R. Bloxham                                          3,177,651    April 13, 1965                                                                             H. R. Lawrence                                      3,167,015-102/103                                                                          Jan. 26, 1965                                                                              B. Smith et al                                      3,049,874    Aug. 21, 1962                                                                              M. R. Morrow et al                                  3,122,887-60/39.82                                                                         March 3, 1964                                                                              B. J. Farmer                                        British Patent No. 850,321 published 5 October 1960.                          ______________________________________                                    

The invention will be more completely understood by reference to thefollowing detailed description, which is to be read in conjunction withthe appended drawings.

BRIEF DECRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C and 1D are curves useful in understanding theinvention.

FIG. 2 is a sectional vew of a part of an internal combustion engineillustrating the invention.

FIGS. 3A, 3B, 4 and 5 are sectional views of exhaust systems inaccordance with the invention.

FIG. 6 is a sectional view of a part of a jet engine illustrating theinvention.

FIGS. 7 and 8 are sectional views of sources for generating ultravioletradiation.

FIGS. 9 and 10 are typical circuit diagrams of electrical circuits usedto energize sources of ultraviolet radiation.

DETAILED DESCRIPTION

It first will be preferable to consider various aspects of photochemicalignition, in general. As noted above, the present invention is directedto the control of the initiation of self-sustaining combustion as wellas the combustion rate and flame propagation velocity in a combustiblemixture. This is achieved by irradiating the mixture with ultravioletradiation. Typically, sensitization of the mixture is not required.Sensitizers are additives foreign to the combustible mixture of interestbut which absorb some of the incident radiation. For example, in thecase of sensitizing by NO₂, the primary photochemical reaction is:

    NO.sub.2 + hv → NO + O.

in the case of mercury, the primary photochemical reaction is:

    Hg + hv → Hg*

followed by

    Hg* + O.sub.2 → Hg + 20.

The mixture which is irradiated may or may not be intermixed fully orpartially with a diluent; for example, in the case of fuel/air as thecombustible mixture, nitrogen is typically present as a major diluent.

As noted above, the present invention involves the photodissociativecreation of a combustion intermediary species by ultraviolet irradiatonof a combustible mixture. In particular, the density of the combustionintermediary species in various parts of the volume occupied by themixture is controlled. For example, in the case of a fuel/oxygenmixture, oxygen molecules can be dissociated into two oxygen atoms whenirradiated with ultraviolet radiation of the proper wavelength. Theresulting photodissociation path and state of the product atoms aredifferent in each of the following wavelength regions:

1. wavelength of photon below 923 A

    o.sub.2 (.sup.3 Σ.sub.g .sup.-) + photon → 20 (.sup.1 S)

2. wavelength of photon 923-1342 A

    o.sub.2 (.sup.3Σ.sub.g .sup.-) + photon → O(.sup.2 P) + O(.sup.1 S)

3. wavelength of photon 1300-1750 A (Schumann-Runge Continuum)

    O.sub.2 (.sup.3Σ.sub.g .sup.-) + photon → O(.sup.1 D) + O(.sup.3 P)

4. wavelength of photon 1750-2000 A (Schumann-Runge Bands)

    O.sub.2 (.sup.3Σ.sub.g .sup.-) + photon → O.sub.2 (.sup.3Σ.sub.u .sup.-) → 20(.sup.3 P)

5. wavelength of photon 2000-2424 A (Herzberg Bands )

    O.sub.2 (.sup.3Σ.sub.g .sup.-) + photon → 20(.sup.3 P)

the process of photochemical initiation of self-sustained combustiondepends primarily on the availability of ultraviolet radiation in theoxygen photodissociation wavelength-regions were strong photonabsorption occurs (e.g., absorption cross sections of 50 cm⁻ ¹ orhigher). Initiation is possible by utilizing other photodissociationwavelength - regions but the process would then require the expenditureof considerably more energy to offset the "weak" absorption of photons.In the strong absorption regions the effective penetration of radiantenergy in air at atmospheric pressure is only in the order of 0.02centimeter. The fact that energy sufficient for photodissociation isabsorbed over such a short path makes it practical to generate largelocal concentrations of oxygen atoms. (It has been determined that anapproximate local concentration of 10¹⁴ oxygen atoms per cubiccentimeter is sufficient to initiate self-sustaining combustion infuel-oxygen-diluent mixtures of practical interest.) However, radiantenergy in these strong absorption regions does not have the penetrationneeded to efficiently affect the combustion process beyond in theinitiation zone and control the combustion rate and flame propagationspeed at in-depth zones of the mixture.

FIG. 1A illustrates the advantages which may be achieved byphotochemical ignition. The ordinate of the curve represents therelative minimum ignition energy which must be achieved to ignite acombustible mixture. The abscissa of the curve is the normalizedair/fuel ratio (normalized with respect to the stoichiometric air/fuelratio). It will be noted that the thermal (spark) ignition curverepresents significantly higher relative minimum ignition energies thandoes the photochemical ignition curve. In fact, the relative minimumignition energy in the case of photochemical ignition is almostconstant, while the required thermal (spark) ignition energy increasesrapidly as the normalized air/fuel ratio increases.

It will be noted from FIG. 1A that, with photochemical ignition, it ispossible to utilize much higher ratios of air to fuel than heretoforepossible. As is known, the use of fuel-lean combustible mixtures greatlyreduces the generation of pollutants from the combustion process.

FIG. 1D illustrates the relation between relative minimum ignitionenergy and pressure for normal (spark) ignition and photochemicallyinduces ignition and demonstrates that the latter can be achieved atmuch lower pressure than the former without the attendant severeincrease in required ignition energy.

FIG. 1B illustrates the combustion enchancement that is possible throughthe use of photochemical ignition. The curve shown in that figure plotsreactant mixture temperature (the ordinate) versus the log ofconcentration of combustion intermediary species (the abscissa). Thepoint designated 20 represents the normal condition of a combustiblemixture at ambient temperature. The curve 22 represents, for thatmixture, the transition between a stable (non-ignited) mixture andcombustion in the mixture. It will be noted that, as the concentrationof a combustion intermediary species increases (e.g., the concentrationof oxygen atoms in a fuel/oxygen mixture increases), the temperature atwhich combustion initiation takes place is reduced. The curve 24represents the transition from the point 20 followed during thermalignition of the mixture. The dashed curve 26, on the other hand,represents the transition in the mixture that is possible throughphotochemical processes. That is, as the mixture is irradiated withultraviolet energy, the concentration of a combustion intermediaryspecies (for example, oxygen atoms) is increased, without a concomitanttemperature change. It is possible, with sufficient irradiation ofultraviolet energy, to achieve ignition. In such a case the incidentultraviolet radiation must be sufficient to achieve the concentrationrepresented by point 28 on the curve 22. On the other hand, the incidentultraviolet radiation may be sufficient only to achieve the pointdesignated 30 in FIG. 1B, for example. In such a case, ignition is notachieved, but the mixture is in a stable condition which is closer toignition than it was before irradiation. Ignition may be achieved fromthe point 30 in one of two ways. First, further incident ultravioletradiation may impinge upon the combustible mixture so that the mixturefollows the path 26 and achieves the point 28 on the curve 22.Alternatively, the temperature of the mixture may be raised so that themixture follows the path designated 32 in FIG. 1B. This latter effectmay be achieved by a spark discharged within the mixture, for example.Ignition is achieved when the temperature is raised slightly to thevalue indicated by the intersection of the curves 32 and 22. It will benoted that the temperature rise from the point 30 to achieve ignition ismuch smaller than the temperature rise along the path 24 from the point20.

FIG. 1B thus illustrates the enhancement of combustion that may takeplace by irradiating a combustible mixture with ultraviolet energyeither to achieve ignition by equaling or exceeding the point 28 orenhancing the ignition process by reaching the point 30, for example. Inthis regard, it will be noted that, for enhancement to occur, theincident ultraviolet radiation must be sufficient to provide aconcentration of combustion intermediary species (oxygen atoms, forexample) within the range designated 34. This range is represented by aconcentration of combustion intermediary species that is greater thanthe concentration existing at the thermal ignition temperature of themixture (the thermal ignition temperature is characterized by theintersection of the curves 24 and 22) but less than the concentrationrequired for photochemical initiation of combustion at the temperatureof the mixture (this latter concentration is represented by theintersection of the curves 26 and 22). It will be appreciated that, ifthe radiant ultraviolet energy is only sufficient to produce aconcentration of combustion intermediary species that is less than orthe equivalent of that which would exist at the thermal ignition point,then a temperature increase to the thermal ignition temperature willstill be required before ignition can occur. Thus, no obvious gain isachieved by irradiating the mixture with such a minimal amount ofultraviolet energy.

It has been found that the creation of a combustion intermediary species(e.g., oxygen atoms) varies throughout a fuel/oxidizer mixture, and foreach zone in the mixture is dependent upon the distance of that zonefrom the source of ultraviolet radiation. The concentration of oxygenatoms, for example, is greatest near the source of ultraviolet energy,and decreases as the distance from the source increases. Thus, theintensity of penetrating ultraviolet radiation may be selected to createthe desired number of atoms at various locations within a combustiblemixture, for example, to precondition for ignition in various locationsand to achieve ignition in others.

It has also been found that different wavelength regions areinstrumental in creating atomic oxygen species, for example, at variousdistances from the source of ultraviolet radiation. Thus, the spectralradiant output of one or more radiation sources may be varied to achievedesired ignition preconditioning and ignition characteristics in amixture. The attached Table 1 indicates, for a representative mixture,the contribution to the creation of oxygen atoms for various zones ofdifferent wavelength regions.

                  TABLE 1                                                         ______________________________________                                        Fractional Contribution of Indicated Wavelength Region                        in Oxygen Formation for Various Distances from a                              UV Grade Sapphire Window and a 300 Torr                                       Stoichiometric Hydrogen-Oxygen Mixture                                        ______________________________________                                                distance from window (cm)                                             wavelength                       x=                                           region (A)                                                                            x=0     x=0.01  x=0.30                                                                              x=3.0 x=30.0 300.0                              ______________________________________                                        1400-1450                                                                             0.099   0.0788  --    --    --     --                                 1450-1500                                                                             0.214   0.181   --    --    --     --                                 1500-1550                                                                             0.241*  0.228*  --    --    --     --                                 1550-1600                                                                             0.202   0.216   0.0097                                                                              --    --     --                                 1600-1650                                                                             0.140   0.163   0.104 --    --     --                                 1650-1700                                                                             0.0698  0.0878  0.382 --    --     --                                 1700-1750                                                                             0.0316  0.0413  0.436*                                                                              0.136 --     --                                 1750-1800                                                                             0.0026  0.0035  0.0610                                                                              0.734*                                                                              0.437* --                                 1800- 1850                                                                            --      --      0.0046                                                                              0.099 0.403  0.315*                             1850-1900                                                                             --      --      --    0.0157                                                                              0.0804 0.296                              1900-1950                                                                             --      --      --    0.0034                                                                              0.0177 0.0842                             1950-2000                                                                             --      --      --    --    0.0082 0.0403                             2000-2050                                                                             --      --      --    --    0.00720                                                                              0.0355                             2050-2100                                                                             --      --      --    --    0.00738                                                                              0.0364                             2100-2150                                                                             --      --      --    --    0.00755                                                                              0.0373                             2150-2200                                                                             --      --      --    --    0.00737                                                                              0.0366                             2200-2250                                                                             --      --      --    --    0.00705                                                                              0.0350                             2250-2300                                                                             --      --      --    --    0.00613                                                                              0.0306                             2300-2350                                                                             --      --      --    --    0.00485                                                                              0.0242                             2350-2400                                                                             --      --      --    --    0.00352                                                                              0.0176                             2400-2450                                                                             --      --      --    --    0.00228                                                                              0.0114                             Total   1.0000  0.9994  0.9973                                                                              0.9881                                                                              0.99963                                                                              1.0001                             ______________________________________                                    

The above principles find application in an automotive engine, forexample, as shown in FIG. 2. A combustible mixture is supplied tocombustion chamber 40 via intake manifold 42. Exhaust gases flow fromthe combustion chamber 40 through exhaust manifold 44. Valves 46 and 48control the flow of gases into and out of the combustion chamber. Thetype of engine shown is one including a reciprocating piston 50,although this type of engine is chosen as being representative only. Aconventional spark plug 52 is utilized, as well as a source 54 ofultraviolet energy. The source 54 has been shown only schematically.More detailed examples are shown in FIGS. 7 and 8, to be describedlater. The source 54 includes a window 56 which forms a part of thecylinder wall 58 of the combustion chamber. Ultraviolet radiation istransmitted through the window 56 into the combustible mixture withinthe combustion chamber 40.

In operation, the source 54 of the ultraviolet radiation conditions thecombustible mixture within the combustion chamber 40 typically byproviding a concentration of oxygen atoms corresponding to the point 30in FIG. 1B. At the appropriate time in the cycle of the engine, when thepiston 50 is at or near top-dead-center, a spark is generated in themixture by the spark plug 52, causing the mixture to traverse the path32 in FIG. 1B and thus to experience ignition and combustion of theentire mixture.

The spark plug 52 could be replaced by another source of ultravioletradiation similar to the source 54, in which case ignition could beachieved by photochemical means entirely. In other words, operationalong the curve 26 of FIG. 1B to the point 30 could be achieved by theultraviolet source 54, and operation along the same curve from the point30 to the point 28 could be achieved by the second ultraviolet soucereplacing the spark plug 52. A single ultraviolet source (as the source54) could be employed to achieve ignition. In any event, operation ofsuch an engine at air/fuel ratios greater than 19 and exhaust gasrecycling back into the combustion process of from 0 to 13% isachievable, resulting in fuel savings and substantial reductions ofNO_(x) emissions.

Application of the above principles to exhaust systems is shown in FIGS.3A and 3B. In FIG. 3A, exhaust system 60 includes a source 62 ofultraviolet radiation positioned therein so that the exhaust gases flowthereabout. The source 62 may typically comprise electrodes 64 and 66spaced from each other and positioned within a cylindrical envelope 68which may be of sapphire or other similar ultraviolet radiationtransmitting material and which is filled typically with an inert gas. Asteady-state or pulsed electrical discharge between the electrodes 64and 66 results in the generation of ultraviolet energy which istransmitted through the envelope 68 into the gaseous exhaust mediumsurrounding the source 62. Further completion of combustion in theexhaust gas medium is thus possible by the generation of combustionintermediary species created by the ultraviolet radiation.

In FIG. 3B, exhaust system 60 includes a source 70 in the form ofannular electrodes 72 and 74 which encircle the exhaust system. In thiscase the wall structure of the exhaust system includes a cylindricalwindow 76 of ultraviolet radiation-transmitting material to transmit theradiation from the electrodes 72 and 74 into the exhaust medium. Outerenvelope 78 may be of any suitable material, which may be opaque ortransparent to ultraviolet radiation.

FIG. 4 shows an alternative arrangement in an exhaust system 80. Source82 has been shown schematically, and may be the same as source 70 inFIG. 3B, for example. Source 84 is also shown schematically and may bethe same as source 70 shown in FIG. 3B or may, for example, take theform of one of the sources shown in FIGS. 7 and 8 to be described inmore detail below. The flow of exhaust gas is as shown by the arrows inFIG. 4, and hence it is apparent that radiation from the source 84impinges upon and penetrates the exhaust gases first, followed byradiation from the source 82. The radiation from the source 84 may, forexample, precondition the exhaust gases to a point equivalent to thepoint 30 in FIG. 1B. Radiation from the source 82 may then achievecombustion in the exhaust gas mixture as by causing the mixture totraverse the path between the points 30 and 28 in FIG. 1B. The sources82 and 84 may also be operated to provide radiation at selectedwavelengths, which may differ from each other, to be described in moredetail below, to precondition or condition the exhaust gas mixture atvarious zones within the mixture, also to be described in more detailbelow.

FIG. 5 shows an exhaust system 90 including a plurality of sources 92spaced along the inside of the system, and a source 94 located along thecentral axis of the system. The sources 92 and 94 may take the form ofthe source 62 shown in FIG. 3A, for example. In any event, the sources92 and 94 irradiate the exhaust mixture with ultraviolet energyconditioning that mixture so as to provide for combustion therein,thereby reducing and removing pollutants.

FIG. 6 illustrates the application of photochemical ignition to a jetengine (typically an aircraft engine). Fuel is supplied by a fuel spraynozzle 96. Primary air is supplied which flows as shown by the arrowsabout the nozzle 96. Secondary and cooling air flows between outer wall98 of the combustion chamber and inner liner 100. A source 102 ofultraviolet radiation is positioned within the wall 98 and extends intothe combustion chamber through the liner 100. The source 102 may takethe form, for example, of the sources shown in FIGS. 7 and 8 to bedescribed below. A plurality of such sources may be utilized as shown byassemblies 104 and 106 drawn in dashed line. One or more of the sources102, 104 and 106 may be ultilized to precondition the combustiblemixture as well as to ignite and complete combustion therein. Thesources may provide radiation of varying intensities at varyingwavelengths penetrating selected zones within the mixture to enhance thecombustion process, and may be timed to operate together orsequentially, for example.

FIG. 7 shows a representative source of ultraviolet radiation. Thesource is comprised of an annular electrode 110 which may be made oftungsten, for example. The electrode is press fitted into a tubular body112 which may be of steel or kovar, for example. Kovar is comprisedgenerally of 29% nickel, 17% cobalt, and 54% iron. A ring 114 (e.g.,copper) may be employed to secure the press fitting of the electrode 110to the body 112 and to provide for good electrical conductiontherebetween. A cup 116 (e.g., kovar) is secured by braising or asimilar procedure to the body 112. A window 118 is secured by braisingor similar securing technique to the cup 116. The window 118 may be ofsapphire, for example, or other suitable material for the transmissionof ultraviolet energy. The outer portion of the body 112 may bethreaded, as at 112a to secure the entire assembly into a wall structurein use. For example, the threaded portion 112a may be utilized to securethe source 102 within the wall structure 98 of the jet engine shown inFIG. 6 and described above.

The source shown in FIG. 7 includes an insulating liner 120 which may beof alumina or other similar insulating material. A central electrode 122(e.g., tungsten) is included which is rod-shaped, the tip portion ofwhich is positioned inside of and coaxially with the annular electrode110. A cylindrical conductive member 124, which may be of molybdenum,for example, is secured to the central electrode 122. An insulatingsleeve 126 (e.g., alumina) surrounds the rear portion of the conductivetube 124. A sealing piece 128 (e.g., kovar) seals the insulating sleeve126 to the body member 112. At the right-hand end of the assembly, theconductive tube 124 has press fitted thereto a tapered pin 130 (e.g.,stainless steel, copper and lead coated). A threaded connector 132(e.g., kovar) is secured (by braising, for example) to the insulatingsleeve 126 and the conductive tube 124. An end cap 134 (e.g., brass) isthreaded onto the connector 132 and includes an end button 136 (e.g.,tungsten brazed to cap 134) which makes electrical contact with thethreaded connector 132 through cap 134. Electrical contact is thusestablished with the central electrode 122 by way of the conductivebutton 136 and with the annular electrode 110 via the body member 112.

In assembling the source shown in FIG. 7, the spaces between theelectrodes 110 and 122, between the insulating insert 120 and conductivetube 124, and inside the conductive tube 124 are filled with an inertgas under pressure. Typically, the gas may be Xenon, at a pressure of200 psia. The gas is introduced into these spaces during assembly of thesource prior to the application of the sealing piece 130 and thecap-button assembly 134-136. Openings 124a in the conductive tube 124permit the gas to flow to the interior region between the electrodes 110and 122.

A source as just described has been employed and operated with 0.15joule of energy in igniting a standard mixture at 300 torr (astoichiometric mixture of two parts of hydrogen and one part of oxygen).

Sapphire has been found to be a suitable window material fortransmission of ultraviolet radiation. The attached Table 2 gives thetransmission characteristics of various materials. While sapphire ispresently the most desirable window material, because of its hardness,scratch and chemical-attack resistance, thermal expansion coefficient,and temperature capability, other materials may suffice for variousapplications.

                  TABLE 2                                                         ______________________________________                                        Low Wavelength Cut-off of Various Far-UV Transmitting Materials               ______________________________________                                        Material       Formula         Cut-off A                                      ______________________________________                                        Lithium Fluoride*                                                                            LiF             1040                                           Magnesium Fluoride*                                                                          MgF.sub.2       1100                                           Calcium Fluoride*                                                                            CaF.sub.2       1250                                           Strontium Fluoride*                                                                          SrF.sub.2       1300                                           Sodium Fluoride*                                                                             NaF             1300                                           Barium Fluoride*                                                                             BaF.sub.2       1345                                           Sapphire (UV Grade).sup.(1)                                                                  Al.sub.2 O.sub.3                                                                              1400                                           Cultured Quartz.sup.(2)                                                                      SiO.sub.2       1450                                           Potassium Fluoride*                                                                          KF              1600                                           Suprasil.sup.(3)                                                                             SiO.sub.2 (amorphous)                                                                         1600                                           Lucalux.sup.(4)                                                                              Al.sub.2 O.sub.3 (polycrystalline)                                                            1700                                           ______________________________________                                         Notes:-                                                                       *Softening points between 400 and 600° C - water soluble and           hydroscopic.                                                                  .sup.(1) Melting point 2040° C, loses some inertness at about          1700° C.                                                               .sup.(2) Melting point 1800° C, crystal change α to β a     550° C - transmission comparable with UV grade sapphire above abou     1500 A.                                                                       .sup.(3) Melting point 1800° C, devitrification starts 1000.degree     C - transmission better than sapphire above 1680 A.                           .sup.(4) Melting point 2040° C - transmission less than 20% over       entire region of interest.                                               

In many instances it is desirable to additionally transmit ultravioletradiation of wavelengths of less than 1450 Angstroms. Since mostsuitable materials for the window 118 of the source of FIG. 7 absorbwavelengths below 1450 Angstroms, as is evident from Table 2 above, thesource of FIG. 8 may be useful in supplying such wavelengths of energy.The source is similar in features and materials to that shown in FIG. 7,except that it completely lacks a window. The source includes a centralelectrode 140 attached to tubular member 142 which extends rearwardly(to the right). The tube 142, which is conductive, is surrounded by anonconductive sleeve 144. The sleeve 144 and tube 142 terminate in athreaded member 146 which includes gas ports 146a therein. A retainingpiece 147 (e.g., kovar) is attached to the insulating member 144. Anannular electrode 148 is included at the left-hand end of the assemblyand held by a threaded cap 150 which is threaded onto threaded endportion of body member 152. It will be noted that the central electrode140 is coaxial with the annular electrode; however, the tip portion ofthe central electrode is not within the annular electrode but is spacedto the rear (to the right) thereof. An insulating sleeve 154 is securedto the inside of the body member 152. The right-hand end of the bodymember 152 is threaded and receives a gas supply and electrical couplingadapter member 156. The member 156 includes a gas inlet 158 throughwhich gas flows as shown by the arrows in FIG. 8. An insulating sleeve160 is secured to the inside of the gas supply member 156. Inside theinsulating sleeve 160 is an insert 162 (nylon, for example). End cap 164and end button 166 are similar to the like components 134 and 136 inFIG. 7. An O-ring 168 is included for sealing purposes.

Gas flows as shown by the arrows through the inlet 158 and thence intothe interior of the conductive tube 142, passing outwardly throughopenings 142a thereof and past the central electrode 140 and through theopening in the annular electrode 148. This gas which flows, typicallyargon, passes into the combustion chamber with which the ultavioletsource is associated at a pressure about the same as that within thechamber. The gas passing into the combustion chamber does not adverselyaffect the combustion process; however, it has the advantage ofisolating the electrodes 140 and 148 from the combustion process whileat the same time transmitting the desired wavelengths of energy belowabout 1450 Angstroms.

FIG. 9 shows an electrical circuit used for energizing the ultravioletsource of FIG. 7. Power source 180 generates a DC potential whichenergizes capacitor 182 which stores a charge thereon. Normally theultraviolet source 184 is non-conductive. It is rendered conductive bythe application of a suitable ionizing potential from a triggertransformer 186. The transformer receives a signal from trigger andtiming circuitry 188 which may be an automotive timing distributor, forexample, or a timed circuit set to provide a signal at a givenrepetition rate, suitable for aircraft ignition, for example. Whenever asuitable ionizing potential is developed by the transformer 186, thesource 184 is ionized rendering it conductive, permitting the capacitor182 to discharge through the source. The discharge of the capacitorresults in a "flashing" of the source, generating light output in theultraviolet spectrum.

The circuitry of FIG. 10 is suitable for energizing a windowless sourceof the type shown in FIG. 8. The circuit of FIG. 10 is the same as thatof FIG. 9, except in this case a trigger control gap 190 is employed.This gap serves the function of providing a constant, high voltage blockwhich prevents the capacitor 182 from discharging into the ignitionsource 184' until a signal is received from the trigger transformer 186.This is necessary when a windowless source is used since the sourcepressure and hence the source breakdown characteristics will vary as theambient pressure to which it is exposed is varied, thus resulting iserratic or incorrect operation. Major losses will occur in transferringenergy from the capacitor 182 to the ultraviolet source 184' through thetrigger gap 190. This loss or inefficiency is avoided by the normal,hermetically sealed source of FIG. 7. Typically, 50% of the energytransferred from the capacitor could be lost in the trigger gap 190.However, such a gap is required when the ultraviolet source utilizes anopening, as in FIG. 8, without any solid barrier isolating theelectrodes from the combustion environment.

The following examples will illustrate and further explain theinvention.

EXAMPLE 1

A stoichiometric hydrogen-oxygen mixture (2H₂ +O₂) is utilized at apressure of 300 torr and temperature of 300° K (27° C). The mixtureeither has zero velocity (stationary) or is flowing at a velocity lessthan about 250 cm/sec. Before ignition and burning occurs, approximately3.24 × 10¹⁸ oxygen molecules/cm³ and 4.91 × 10⁻ ²² oxygen atoms/cm³ willbe present at ambient temperature.

Normal thermal ignition would require a mixture temperature increase of553° K to a temperature of about 853° K at which point approximately2.18 × 10⁶ oxygen atoms/cm³ would be present.

An ultraviolet source is used to precondition the mixture before anignition source is applied. This source is of the type shown in FIG. 7and has the characteristics: sapphire window, window aperture of 1/4diameter and radiant power flux in the vacuum ultraviolet of 10microwatts/cm² A at about 1500 A impinging on the window -- in thesteady operating mode. Steady oxygen atom concentrations of about 2.78 ×10¹³ oxygen atoms/cm³ are developed at the ignition site (near thesource) prior to the application of an ignition pulse, while at areasonable distance into the mixture away from the source, say 5cm,steady oxygen atom concentrations of about 4.72 × 10¹⁰ oxygen atoms/cm³are developed prior to initiation of burning. These values are achievedin about 2.5 milliseconds after turn-on (initiation of operation).

A superimposed spark discharge initiates combustion; however, theignition kernel is initiated with a temperature increase to only 395° Kinstead of the usually needed 853° K. Further, at a point 5cm away fromthe window combustion begins when only a 563° K temperature is reachedinstead of the usual 853° K. Alteration of the radiant power of thesource results in control of the initiation and burning requirementwithin the ignition kernel and at in-depth (away from the window)locations of the mixture.

For an ultraviolet source of the type shown in FIG. 7 being run steadystate, if it is assumed that the conversion efficiency of stored energyinto ultraviolet light for the steady operating mode in one tenth thatfor the pulsed operating mode, then the source output power flux in theultraviolet for this example represents a required input power of about86.8 watts.

EXAMPLE 2

The same conditions as in Example 1 apply, except that the ignitionsource is provided by superimposing an ultraviolet flash. This flash isgenerated by the same source used to provide the continuous or steady,low level ultraviolet flux -- although a second, separate radiant sourcecould be employed as well. The output characteristics of thephotochemical source are as follows: steady state ultraviolet energy ofabout 10 microwatts/cm² A; superimposed flash of an additional 1690microwatts/cm² A for about 100 microseconds.

After the short duration, high power flash, the oxygen atomconcentration near the source rises to about 8.42 × 10¹⁴ atoms/cm³ whichexceeds the critical concentration for ignition without the need for anattendent temperature rise, thus initiating combustion.

At a point 5cm into the mixture measured (from the window) the oxygenatom concentration rises to about 1.43 × 10¹² atoms/cm³. Consequently,combustion begins at this point when only a 475° K temperature isreached instead of the usual 853° K.

For a source of the type shown in FIG. 7, the superimposed radiant pulserepresent an input energy dump of only about 150 millijoules.

EXAMPLE 3

The same conditions and results as in example 2 apply, except thatsequential pulsing is employed as follows: steady state ultravioletenergy to precondition the mixture is provided by a series of discretepulses of energy about 20 microwatts/cm² A, each of a duration of about250 microseconds spaced from each other by about 250 microseconds. Theseries lasts for a period of about 2.5 milliseconds (about 5 discretepulses) followed in about 250 microseconds by a flash (pulse) ofultaviolet energy of about 1700 microwatts/cm² A lasting for about 100microseconds to initiate combustion in the preconditioned mixture.Alternatively, the last-mentioned ultraviolet pulse is replaced by aspark pulse to achieve ignition in the preconditioned mixture.

For a source of the type shown in FIG. 7, each of the 5 low levelultraviolet pulses represents input dumps of only about 4.35 millijoulesper pulse. This is followed by a 150 millijoule final-ignitionultraviolet pulse.

EXAMPLE 4

The same conditions apply as in Example 1, except that the ultravioletsource is replaced by two separate ultraviolet sources both atapproximately the same location. Each of the sources is optimized to aparticular wavelength region as follows: Source No. 1 is optimized inthe 1500 A to 1550 A region (ignition) such that 5% of the total energywhich was originally spread over the ultraviolet region of interest isnow concentrated in the 1500 A to 1550 A region -- see FIG. 1C. SourceNo. 2 is optimized in the 1750 - 1800 A region (in-depth combustionenhancement) in the same manner -- see FIG. 1C. In FIG. 1C, the dashedcurve represents a non-optimized source. The area between thenon-optimized and optimized curves represents the energy shifted intothe optimized region. Optimization of a source of the type shown in FIG.7 is achieved by varying gas pressure, gas type, electrode spacing andconfiguration, for example.

Before optimization, the original non-optimized source required about150 millijoules input energy for ignition (creation of 8.29 × 10¹⁴atoms/cm³ near the window). This also resulted in in-depth combustionenhancement, for example -- at 5cm into the mixture, about 1.41 × 10¹²atoms/cm³ were created, resulting in the lowering of the ignitiontemperature from about 853° K to about 475° K. The same ignition andenhancement effects are achieved by pulsing both of the optimizedsources instead of a single non-optimized source. Identical effects areobtained by pulsing source No. 1 at 38 millijoules and source No. 2 at33 millijoules. Thus the total pulse energy required is reduced from 150millijoules to about 71 millijoules.

If only optimized source No. 1 were used, ignition would be achievedwith the expenditure of only about 48 millijoules (instead of the 71millijoules noted above). However, the enhancement effect would bereduced, with only about 4.28 × 10¹¹ oxygen atoms/cm³ being created at adepth of 5cm. This results in lowering the ignition temperature from853° K (without enhancement) to about 515° K. Thus the equivalent of 40°K enhancement effect is lost compared to the above example but a savingsin input energy of about 32% is realized.

EXAMPLE 5

The same conditions as in example 1 apply, except that combustion isinitiated at one end of a tubular chamber 5cm in diameter and also thecombustion process is affected at a distance 5cm away from the ignitionsite on the center line of the chamber. These requirements are met byusing two sources: Source No. 1 is located near the ignition site on theaxis of the tubular chamber, and is optimized in the 1500 - 1550 Aregion for ignition as per the previous example. Source No. 2 is located5cm away from the ignition site in the side wall of the chamber. Thislocates it 2.5cm away from the desired enhancement area. Source No. 2 isoptimized in the 1750 - 1800 A region as per the previous example.

Ignition is achieved by pulsing source No. 1 at a 48 millijoule inputlevel (assuming typical efficiencies of the source of the type shown inFIG. 7). When the in-depth enhancement goal at the 5cm distance is areduction of ignition temperature from the normal 853° K to 475° K,source No. 2 is supplied at a pulse energy level of only about 9.5millijoules. Thus the total energy consumed is only about 57.5millijoules compared with 71.0 millijoules if both sources were at theignition site as in example 4.

EXAMPLE 6

The same conditions as in example 2 apply, except that two sources areused. One source provides a steady or quasi-steady (duration times ofabout 2 to 3 milliseconds) radiant flux of about 10 microwatts/cm² A.The second source is sequenced to pulse after time periods equal to orgreater than 2 to 3 milliseconds and provides about 1700 microwatts/cm²A.

Each source may be tuned for a particular wavelength distribution.

SUMMARY

The present method depends on controlling the density of a combustionintermediary species (e.g. oxygen atoms) in various parts of the volumeoccupied by a fuel/oxidizer mixture. This is accomplished by causingultraviolet irradiation to impinge on the mixture causing directphotodissociation of oxygen molecules into oxygen atoms, for example.

Photodissociation of oxygen molecules in wavelength regions where weakphoton absorption occurs [for example, in the Schumann-Runge band system(1750 - 2000 A) or the Herzberg band system (2000-2424 A)] results inpenetration of radiant energy in air at atmospheric pressure up toseveral meters or more. As a result, these weaker untraviolet radiationabsorption regions may be used to control the combustion rate and flamepropagation speed by dissociating a number of oxygen molecules within afuel-oxygen-diluent mixture, thus creating oxygen atoms that are presentwhen the combustion process engulfs an in-depth volume of the reactantmixture. The number of oxygen atoms created in this manner prior to thearrival of a flame front can be sufficient for controlling the rate ofcombustion and flame front propagation speed, but not necessarilysufficient to produce initiation of combustion throughout the volume ofthe fuel-oxygen-diluent mixture.

An ultraviolet radiation source (or sources) is used to provideultraviolet radiation in regions desired. Radiation in various regionsof interest can be generated simultaneously or timed at intervals. Theweak penetration-strong absorption regions are primarily used ininitiating the self-sustaining combustion process. The strongpenetration-weak absorption regions are primarily used in controllingcombustion processes. It is clear that by controlling the intensity inthe photochemical enhancement regions as well as the timing ofirradiation relative to the initiation of combustion, photochemicalinitiation and control of initiation of the combustion process, as wellas control of the combustion rate and flame propagation speed, can beobtained.

Although the primary mechanism for initiation and control of combustionis the photodissociation of oxygen molecules into two oxygen atoms, itshould be noted that during the irradiation process other species may beformed within the same volume of the fuel-oxygen-diluent mixture. Suchspecies could be excited oxygen molecules, excited molecules and atomsof the fuel or the diluent, or the intermediary species produced by theensuing reactions. The presence of each species constitutes the creationof a combustion intermediary species.

The techniques described in the present application may be used toeliminate or reduce some or all of the various limitations inherent inother combustion initiation processes mentioned above (e.g., spark), aswell as to provide for the control of the rate of combustion and thespeed of the propagation of the flame that is generated. Photochemicalinitiation of combustion control of initiation of combustion and controlof combustion by means of irradiation by selected ultraviolet radiationcan be used effectively over a wide range of pressure, temperature, flowconditions, turbulence, fuel type and stoichiometry. Furthermore, thetechniques can be used to reduce the delays in the combustion initiationprocess and enhance mixing fuel-oxygen-diluent reactant mixtures andcontrol combustion instabilities.

It will be evident that many different techniques as outlined above arepossible. For example, a continuous, pulsed or modulated power supplymay be utilized, enabling the ultraviolet radiation source to operate invarious modes such as: a single flash of appropriate duration; thesource being on continuously at a level of intensity sufficient toprecondition the fuel-oxygen-diluent mixture in depth, and then a flashsuperimposed to initiate combustion; sequential flashes of appropriateinterval and duration; a source continuously on and capable ofinitiation of combustion. Furthermore, there may be more than oneultraviolet radiation source, such as: a number of ultraviolet sources,each tuned or optimized to a separate wavelength region of radiation; anumber of ultraviolet sources directed to irradiate various parts of afuel/oxidizer mixture with variation in spectral distribution andintensity of the radiant output; a number of ultraviolet sources used insequence or phased according to a preselected timing sequence thatprovides varying intensities and spectral distributions of ultravioletradiation to the reactant mixture at varying times.

There may also be varied geometric relationship between a radiationsource and a combustion chamber, such as one source to one chamber, onesource for many chambers, one chamber with many sources.

An important realization of the present invention is that increasing theinput energy results in a pronounced effect on the reaction frontpropagation velocity. For example, a stoichiometric hydrogen-oxygenmixture at 300 torr pressure and room temperature was exposed to radiantenergy levels which resulted from 200 to 300 joule energy inputs. Thereaction front arrival time at photo-cells positioned at 7.52 cm and12.52 cm from the radiant source window (ultraviolet grade sapphire) wasinterpreted to demonstrate that an increase in input energy by a factorof 10 results in reducing the reaction front arrival time approximately50%. It is expected that a similar reduction in ignition delay can bebrought about by increasing the input energy. The proposal thatenhancement in depth is created by the generation of oxygen atoms isthus supported since, in the region discussed, ten times the normaloxygen atom concentration is generated for ten times the amount ofenergy. Increasing the input energy results in an increase in reactionfront propagation velocity.

It should be noted that there are no presently known window materialsthat transmit photons below 1000 A (lithium fluoride windows transmitradiation down to about 1040 A). As a result, if radiation below the LiFultraviolet cut-off wavelength is desired, the operation of anultraviolet source has to be windowless by necessity. Operation in thismode is as discussed in connection with FIG. 8, and provides lowerwavelength radiation than that normally available from sealed radiantsources. This results in additional contributions to the importantoxygen photodissociative reactions, above that normally possible whensources with windows are used. Further, windowless operation avoids thepotential problem of window transmission loss due to contamination,either internal or external. In terms of practical industrial materialsavailable, sapphire (which transmits radiant energy down to about 1430A) and special quartz (which transmits radiant energy down to about 1550A) provide the lowest usable wavelengths. Sapphire is preferable toquartz because of its lower ultraviolet cut-off limit. Lower wavelengthtransmitting windows (such as lithium fluoride) do not have thenecessary structural strength, have high temperature limitations, andare hydroscopic; all of these features being undesirable in mostpractical situations.

An important aspect of the present invention relates to the use ofphotochemical ignition and combustion control within the combustionchamber of an internal combustion device for the purpose of reducing oreliminating engine exhaust pollutants, particularly nitric oxides.Pollution control is achieved by taking advantage of the differentcombustion characteristics that are possible with the photochemicalmethod compared to the spark method. Direct control within thecombustion chamber is accomplished by either initiating a rapidcombustion front or ignition of fuel-lean mixtures.

Experiments have indicated that acceptable pollution controls,particularly of nitric oxides, can be achieved by providing for reliableand positive ignition and combustion of the appropriate fuel-leanmixtures, with or without exhaust gas recycle. However, ignition offuel-air mixtures by means of spark sources becomes increasinglydifficult and eventually impossible when the reactant mixture is madeprogressively more fuel-lean. Photochemical ignition and combustioncontrol of reactant mixtures does not experience the same difficulties.

A comparison between the two methods, shown in FIG. 1A, illustrates thevastly different characteristics that can be obtained by using thephotochemical technique. Energy requirements for spark ignition offuel-lean mixtures increase drastically compared to energy requirementsfor photochemical ignition as the mixture is made leaner.

The increased ignition capability may be used to successfully operateinternal combustion engines at conditions which currently representregions of poor combustion. This would then provide for operation atgreatly reduced NO_(x) levels, either with or without exhaust gasrecycle.

In operation, the spark plug and its associated electronics is replacedby a photochemical ignition source with its electronics, or if notreplaced then accompanied by a photochemical ignition source. The engineis tuned to run fuel-lean with or without exhaust gas recycle.

A second aspect of pollution control within the combustion chamber isbased on the finding that nitric oxide is formed in significantconcentrations only in the postflame reactions following passage of aflame front. That is, it forms at an insignificant rate at temperaturesencountered through the flame front, and only begins to proceed at arapid rate at the high temperatures characteristic of the combustionproducts. Thus, it is possible to describe the formation time of nitricoxide at the combustion products temperature by a characteristic time,τ_(no).

Since τ_(no) is determined by gas-phase chemical kinetics, and thus is astrong function of combustion product temperature (as is the equilibriumnitric oxide concentration), it is susceptible to control throughvariation of fuel-air ratio (as is the equilibrium nitric oxideconcentration).

For example, changing the fuel-air ratio from a fuel-rich to a fuel-leancondition (say equivalence ratio 1.2 to 0.9) results in an increase inτ_(no) from a few msec to approximately 10 msec in gasoline-fueledautomotive engines.

An important aspect of the photochemical control technique is thatignition and flame propagation are affected using the photochemicalignition process as opposed to spark ignition. For example,photochemical ignition of a methane-air mixture can be made to occur atroom temperature in times much less than the time involved in sparkignition and the ensuing propagating reaction front is stronger evenwithout motion in the unburned gas. On the other hand, electric spark(thermal) ignition requires first a large temperature rise above roomtemperature, and propagates a weaker reaction front for similarconditions.

In practice, substitution of a photochemical ignition source for aspark-ignition source should reduce the time required for combustion ofthe fuel charge. This permits retardation of ignition until the pistonis close to top dead center or to the power stroke. Fuel energy releasewill be initiated and controlled photochemically and combustion productswill be generated during a small piston excursion from top dead center,so that engine efficiency will be essentially unaltered. However, sincethe combustion can be timed to occur near or at the start of the powerstroke, significant expansion will take place before very much nitricoxide can be produced.

Still another application of the method of photochemical initiation andcontrol of combustion within the combustion chamber of an engine is toprovide for operation of engines outside their normal limits. Forexample, the design of aircraft combustors are limited by therequirements of spark ignition and normal combustion characteristics.Photochemical ignition permits operation of conditions not normallypossible, thus extending the possible design range.

Yet another application of the method of photochemical initiation andcontrol of combustion is to provide for the treatment of engine orchemical process exhaust gas products. For example, the three mainpollutants from engine emissions are unburned hydrocarbons, carbonmonoxide and oxides of nitrogen. They result as a consequence of theinability of the combusting hydrocarbon-air mixture to maintainequilibrium burning rates during the expansion process in the combustionchamber. Further combustion and burning of these species byphotochemically catalyzing combustion reactions within the exhaustmanifold of the machine is possible due to the enhanced combustioncharacteristics provided by the photochemical method.

The invention, described above, is to be defined by the followingclaims.

What is claimed is:
 1. In a method of controlling the initiation ofcombustion and controlling combustion within an internal combustionengine supplied with a combustible fuel-air mixture, the step comprisingirradiating the mixture with ultraviolet energy in the region belowabout 2450 A and of sufficient intensity to initiate combustion at atemperature below the thermal ignition temperature of the mixture, inwhich the engine is an aircraft engine.
 2. Apparatus for controlling theinitiation of combustion and controlling combustion in a combustiblemixture, comprising a combustion chamber for receiving said combustiblemixture, and a plurality of sources of ultraviolet energy positioned toirradiate the mixture within said chamber with ultraviolet energy, inwhich said combustion chamber is in an aircraft engine.